Manufacture of chromium oxide



Patented July 31, 1934 1,968,599 MANUFACTURE or CHROMIUM OXIDE Jules Emile Demant, Cuise-Lamotte, France, as-

signor to Bozel-Maletra Socit Industrielle dc Produits Chimiques, Paris, France, a corporation of France No Drawing. Application June 13, 1930, Serial No. 461,051. In Grea t Britain May 22, 1930 5 Claims. (CL 23-145) Chromium sesquioxide, CrzOa, is usually manufactured by treating a trivalent chromium salt with an alkali and in calcining the hydroxide so precipitated, according to thefollowing reactions:

The trivalent chromium salt used as starting material can be obtained in two different ways, to wit:

Ferrochromium can be dissolved in a mineral acid dissolving not only chromium but iron also. The liquors thus obtained, however, are soiled with iron which it is diflicult to remove therefrom. Consequently the chromium oxide prepared from said liquors is also more or less impure.

Chromium oxide free from iron can be ad vantageously obtained by the use of a trivalent chromium salt such as Cr2(SO4)3, obtained by reducing an alkali metal chromate such as NazCrOa Said chromate may be, for instance, easily obtained free from iron by oxidizing calcined chromium ore in the presence of sodium carbonate. The alkali metal chromate is reduced in an acid medium according to the following reaction:

The chromium sulfate being free from iron will be useful for preparing chromium oxide according to the preceding reactions 1 and 2.

The reactions 1, 2 and 3 clearly show that five moles of sulphuric acid and an equivalent amount of chromium oxide are essential to produce one mole of chromium oxide and five moles of sodium sulphate. It is to be noted that two moles of NazCOs are involved in the preparation of the alkali metal chromate while three moles of NazCOa are essential in reaction 1 above to convert the Crz(SO4)3 to Cr(OH)3. A too great consumption of acid and of alkali, therefore,

J renders said method very uneconomical.

According to the invention, chromium oxide or valuable substances containing the same are manufactured in a much more convenient and economical manner than in the methods outlined above, by subjecting impure chromium or an alloy or mixture containing chromium, in particular cheap commercial ferro-chrome, to an oxidizing roasting process atan elevated temperature below the temperature of fusion of the metal (l520 (3.), and if, for example, a starting material containing iron is used, by removing the iron afterwards in a suitable way.

Although it is known that ferro-chrome can be oxidized at 1200" C. in the presence of alkalies, this roasting only leads toneutral chromate.

It is also known that ferro-chrome may be oxidized by electrolytic means to chromate or bichromate, or even to chromic acid.

However no known process starting with ferrochrome leads directly to chromium sesquioxide.

The details now known regarding the proper-. ties of chromium and its alloys do not in any way suggest the possibility of converting the chromium to chromium oxide by an ordinary roasting process, especially as chromium by its behaviour is classed with the noble metals. Thus according to Ullmanns Technologie (2nd Edition, p. 380) chromium, particularly when containing carbon, is a non-oxidizable metal. It is in fact known that just because of its properties resembling those of the noble metals it is being used toan ever increasing extent for protective coatings and also for the production of rustless steel wherein the carbon content varies between 0.25% to 1%.

From the hitherto known properties of chromium and its alloys, it is not obvious that the chro mium in such materials containing chromium could be converted directly to chromium sesquioxide; the contrary conclusion would appear more reasonable from this prior knowledge.

By means of the present invention it has been made possible for the first time to manufacture chromium sesqui oxide from the cheap chromium alloys, particularly commercial ferro-chrome, in a technically simple and economical manner, by a roasting process. It is by no means necessary to use very pure materials containing chromium; ordinary cheap commercial ferro-chrome alloys are pre-eminently suitable. Even the high carbon content (over l2%) has no ill effect as the carbon is burnt to carbon dioxide.

It is advantageous to subject the starting material to the roasting process in as fine a state as possible.

The required temperature and duration of roasting depend of course on various factors, such as the fineness and nature of the starting material. In general the rapidity of reaction increases with increasing temperature.

The roasting process may be undertaken in any suitable hearth or muffle furnace, but it is advantageously carried out in a mechanical furnace of the type of a pyrites furnace or in a multistage furnace.

The rapidity of reaction is such as to make it possible to carry out the roasting in a continuous process. Experiments have shown that with the use of ferro-chrome containing 60% Cr and 8 to 10% C., heating the material to 1000" to 1200" C. and stirring it, results at the end of 3 hours in more than of the iron and chromium being oxidized.

It is to be noted that the original high carbon content, with a lower limit of about 1 i to 2%, differentiates the commercial ierro-chrome roasts, as used by the present process, from the constituency of so-called stainless steels, where the carbon contents never exceeds 1%, and is usually much lower.

It is known that chromium sesqui oxide on heating goes over into a form insoluble in acids, and this makes it possible to remove any iron oxide in a very simple way by a treatment with acids Without the loss of any of the chromium.

The complete removal of the iron can be brought about if the roasted product is treated at a high temperature in a reducing atmosphere to reduce the iron oxide. The metallic iron can then be separated, as indicated. before, from the insoluble chromium oxide by a mineral acid. As a matter of fact, chromium oxide is in practice not reduced through reducing gases; carbon reduces it at high temperatures in the electric furnace, as well as alkali metals and aluminum. On the contrary the iron oxides are easily reduced at temperatures at which chromium. oxide is not attacked. When reducing, by means of reducing gases, the above reaction product containing chromium oxides and iron oxides, only the iron oxide is reduced.

Example Commercial ferro-chrome containing about 50-60% Cr and 53-10% C. is roasted in a two stage muiile furnace with agitating devices to about 1000-1200 C. in a strong current of preheated air. After about 12 to 15 hours the oxidation is practically complete. After removing the roasted material it is treated as required by customary methods for removal of the iron.

I claim:

1. A process for the manufacture of chromiun sesqui oxide which consists in roasting finely ground ferro-chromium having a carbon content exceeding 2% with gases containing oxygen at an elevated temperature below the temperature of fusion of the metal.

2. A process of manufacturing chromium sesqui oxide, which consists in roasting a finely ground ferrochromiutn having a carbon content exceeding 2% withgases containing oxygen, at an elevated temperature below the temperature of fusion of the metal, while stirring the mass.

3. A process of manufacturing chromium sesqui oxide which consists in roasting a finely ground ferrochromium, having a carbon content exceeding 2%, with gases containing oxygen, at a temperature of 1000l200 C.

i. A process of manufacturing chromium sesqui oxide which consists in roasting a finely ground ferrochromium, having a carbon content exceeding 2%, with gases containing oxygen, at a temperature of l0O0-l20(i C., while stirring the mass.

5. A process for the manufacture of chromium sesqui oxide which consists in roasting finely ground commercial ferrochromium, having a carbon content exceeding 2%, with gases containing oxygen, at an elevated temperature below the fusion of the metal, while stirring the mass, in heating the reaction product in the presence of reducing gases to reduce the iron oxide contained in said reaction product, and in lixiviating the resulting reduced iron with a mineral acid.

JULES EMILE DEMANT. 

